Multi-chip stack structure and method for fabricating the same

ABSTRACT

A multi-chip stack structure and a method for fabricating the same are provided. The method for fabricating a multi-chip stack structure includes disposing a first chip group comprising a plurality of first chips on a chip carrier by using a step-like manner, disposing a second chip on the first chip on top of the first chip group, electrically connecting the first chip group and the second chip to the chip carrier through bonding wires, using film over wire (FOW) to stack a third chip on the first and the second chips with an insulative film provided therebetween, wherein the insulative film covers part of the ends of the bonding wires of the first chip on the top of the first group and at least part of the second chip, and electrically connecting the third chip to the chip carrier through bonding wires, thereby preventing directly disposing on a first chip a second chip having a planar size far smaller than that of the first chip as in the prior art that increases height of the entire structure and increases the wiring bonding difficulty.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a semiconductor structure andmethod for fabricating the same, and more particularly to a multi-chipstack structure and method for fabricating the same.

2. Description of Related Art

Currently, multi-chip module (MCM) semiconductor packages have beendeveloped to improve performance and capacity of single semiconductorpackages, thereby meeting the demand for electronic products havingsmaller size and higher operation speed. Typically, two or more chipsare disposed in a multi-chip module semiconductor package so as toreduce the entire structure volume of electronic products and improveelectrical performance of the electronic products. In other words, bycombining two or more chips in a single package structure, limit on thesystem operation speed is minimized, and in addition, the multi-chippackage structure can reduce length of connecting circuit between thechips so as to reduce the signal delay and access time.

The multi-chip packages generally have a side-by-side structure, thatis, two or more chips are side-by-side disposed to a common substrate,and electrically connected to the substrate by such as wire bonding.However, the side-by-side structure can lead to a high package cost andlarge package size since the area of the common substrate needs toincrease as the number of the chips increases.

Accordingly, chip stack structures are proposed to overcome the abovedrawbacks. According to the difference designs of the chips, the wirebonding process of the chip stack structures can be slightly different.For example, if bonding pads of the chips are designed to be located atone side, such as flash memory chips or DRAMs (Dynamic Random AccessMemory), the chips are stacked in a step-like manner for facilitatingthe wire bonding process. As shown in FIGS. 1A and 1B, wherein FIG. 1Bis an upper view of FIG. 1A, a multi-chip stack substrate disclosed byU.S. Pat. No. 6,538,331 is shown. A plurality of memory chips is stackedon a chip carrier 10, wherein the first memory chip 11 is disposed onthe chip carrier 10, the second memory chip 12 is stacked on the firstmemory chip 11 and offsets a certain distance from the first memory chip11 such that the wire bonding process for bonding pads of the firstmemory chip 11 is not affected by the second memory chip 12. Further, acontroller chip 13 is stacked on the second memory chip 12. The memorychips 11, 12 and the controller chip 13 are electrically connected tothe chip carrier 10 through a plurality of bonding wires 15.

Further, in order to increase the memory capacity of memory cards, thenumber of the memory chips also needs to be increased. FIG. 2 shows amulti-chip stack structure disclosed by U.S. Pat. No. 6,621,155, whereina plurality of memory chips 21, 22, 23, 24 is stacked on a chip carrier20 in a step-like manner, and a controller chip 25 is further disposedon the memory chips 21, 22, 23, 24.

However, if more and more chips are stacked in the above-described way,the projecting area of the entire structure is continuously increased,when a certain number of the stack layers is reached, the memory chipswill go out of the packageable range. Thus, the area of the chip carriermust be increased to finish the chip stack, which however increases theentire package volume and cannot meet requirement of small size andmultifunctional electronic products.

Also, as planar size of a controller chip is far smaller than that of amemory chip, when the controller chip is electrically connected to achip carrier through bonding wires, the bonding wires will definitelypass over the memory chips located below the controller chip, which caneasily lead to contact of the bonding wires with the memory chips andeven cause short circuit problems. Meanwhile, the wire bonding processbecomes much more difficult.

On the other hand, if the controller chip is disposed to a region otherthan the region for disposing of the memory chips, the use area of thechip carrier is increased.

Referring to FIG. 3, Taiwan Patent No. I255492 discloses anothermulti-chip stack technique, wherein a plurality of memory chips 31, 32is stacked on a chip carrier 30 in a step-like manner and electricallyconnected to the chip carrier 30 through bonding wires 36. Then, abuffer layer 37 is disposed on the memory chips 31, 32 and a pluralityof memory chips 33, 34 is further disposed on the buffer layer 37 in astep-like manner. Thereafter, a controller chip 35 is disposed on thememory chips 33, 34. Thus, the number of the stacked chips is increasedwithout going out of the packageable range.

However, the above-described method still cannot overcome the problemthat the bonding wires of the controller chip contacting the memorychips on the lower side as well as the short circuit problem and wirebonding difficulty. Further, such a method requires long bonding wiresand the wire arc is too high, which accordingly increases the processcost and can easily result in a wire sweep problem.

Meanwhile, the disposing of the buffer layer increases the process costand steps, and also increases the height of the whole multi-chipstructure, thereby making it difficult to be applied in fabricating thintype electronic devices such as Micro-SD cards.

Furthermore, in the above-described fabrication processes, as thecontroller chip is stacked on top of the memory chips, the height of theentire stack structure is adversely affected. In addition, too longbonding wires reduce electrical connection quality. If the number of thestack layers increases, delamination can easily occur on the interface.

Therefore, how to overcome the above-described drawbacks has becomeurgent.

SUMMARY OF THE INVENTION

Accordingly, an objective of the present invention is to provide amulti-chip stack structure and method for fabricating the same, whichallows multi-layer chip stack without increasing the package area andheight.

Another objective of the present invention is to provide a multi-chipstack structure and method for fabricating the same, which reduces thestack height and is applicable in thin-type electronic devices.

A further objective of the present invention is to provide a multi-chipstack structure and method for fabricating the same, which can reducethe wire bonding difficulty and prevent bonding wires from contactingthe chips so as to avoid short circuit problem.

Still another objective of the present invention is to provide amulti-chip stack structure and method for fabricating the same, whichcan reduce the required length of bonding wires and arc height so as toreduce the process cost and prevent the conventional wire sweep problemand strengthen the electrical connection quality.

A further objective of the present invention is to provide a multi-chipstack structure and method for fabricating the same, which can decreasethe number of the stack layers and probability of delamination occurringon the interface.

Another objective of the present invention is to provide a multi-chipstack structure and method for fabricating the same, which can simplifythe fabrication process.

In order to attain the above and other objectives, the present inventiondiscloses a method for fabricating a multi-chip stack structure, whichcomprises: disposing a first chip group comprising a plurality of firstchips on a chip carrier in a step-like manner, disposing a second chipon the first chip on top of the first chip group, wherein the first andsecond chips are electrically connected to the chip carrier throughbonding wires; stacking a third chip on the first chip group and thesecond chip with an insulative film provided therebetween, theinsulative film covering part of the ends of the bonding wire of thefirst chip on the top of the first chip group and at least part of thesecond chip; and electrically connecting the third chip with the chipcarrier through bonding wires. Thereafter, a fourth chip can be stackedon the third chip in a step-like manner and electrically connected tothe chip carrier through bonding wires. Further, an encapsulant can beformed on the chip carrier to encapsulate the chips. Furthermore, theinsulative film has such a thickness that the bonding wires of the firstand second chips are prevented from contacting the non-active surface ofthe third chip.

Through the above-described fabrication method, the present inventionfurther discloses a multi-chip stack structure, comprising: a chipcarrier; a first chip group comprising a plurality of first chipsdisposed on the chip carrier in a step-like manner, wherein the firstchips are electrically connected to the chip carrier through bondingwires; a second chip disposed on the first chip on top of the first chipgroup, wherein the second chip is electrically connected to the chipcarrier through bonding wires; and a third chip stacked on the firstchip group and the second chip with an insulative film providedtherebetween, wherein the insulative film covers part of the ends of thebonding wire of the first chip on the top of the first chip group and atleast part of the second chip, and the third chip is electricallyconnected to the chip carrier through bonding wires.

The multi-chip stack structure further comprises a fourth chip stackedon the third chip, and an encapsulant encapsulating the first chip groupand the second to fourth chips. In addition, the insulative film hassuch a thickness that the bonding wires of the first and second chipsare prevented from contacting the non-active surface of the third chip.

The chips are electrically connected to the chip carrier through a usualwire bonding method or a reverse wire bonding method, wherein, accordingto the wire bonding method, the bonding wires has a ball bond formed toconnect the chip carrier and then a stitch bond formed to connect thechips, thus reducing the arc height and thickness of the insulative filmso as to achieve a much lighter and thinner multi-chip stack structure.

The first, third and fourth chips respectively have bonding pads formedat single side thereof (such as memory chips) and are stacked in astep-like manner. The second chip has bonding pads formed on at leastone side thereof (such as controller chip). Planar size of the secondchip is smaller than those of the first, third and fourth chips.

Therefore, the present invention comprises disposing a first chip groupcomprising a plurality of first chips (memory chips) on a chip carrierin a step-like manner; disposing a second chip (a controller chip) onthe first chip on top of the first chip group, wherein the first andsecond chips are electrically connected to the chip carrier throughbonding wires; using a film over wire (FOW) technique to stack on thefirst and second chips a third chip (a memory chip) with an insulativefilm provided therebetween, wherein the insulative film covers part ofthe ends of the bonding wires of the first chip on top of the first chipgroup and at least part of the second chip; and electrically connectingthe third chip to the chip carrier through bonding wires. Thus, thecontroller chip having a size far smaller than the memory chips isprevented from being directly stacked on the memory chips as in theprior art, thereby decreasing height of the entire structure andavoiding wire bonding difficulty and even short circuit problemoccurring if the bonding wires of the controller chip pass over andcontact the memory chips. Further, as the controller chip is disposedbetween the memory chips instead of on top of the memory chips as in theprior art, the required length of the bonding wires and the arc heightare reduced, thereby saving the fabrication cost, avoiding the wiresweep problem and improving the electrical connection quality.Meanwhile, the number of the chip stack layers is reduced, probabilityof delamination occurring on the interface is reduced and the processcomplexity is simplified. Furthermore, as the third chip (memory chip)is stacked on the first and second chips with an insulative filmprovided therebetween by using a film over wire technique and theinsulative film covers part of the ends of the bonding wires of thefirst chip on the top of the first chip group and at least part of thesecond chip, the need of a conventional buffer layer is eliminated,thereby enabling the entire structure to become much thinner.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are respectively sectional and planar diagrams of amulti-chip stack structure disclosed by U.S. Pat. No. 6,538,331;

FIG. 2 is a diagram of a multi-chip stack structure disclosed by U.S.Pat. No. 6,621,155;

FIG. 3 is a diagram of a multi-chip stack structure disclosed by TaiwanPatent No. I255492;

FIGS. 4A to 4D are sectional diagrams showing a multi-chip stackstructure and a method for fabricating the same according to a firstembodiment of the present invention; and

FIG. 5 is a sectional diagram showing a multi-chip stack structure and amethod for fabricating the same according to a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following illustrative embodiments are provided to illustrate thedisclosure of the present invention, these and other advantages andeffects can be apparent to those skilled in the art after reading thedisclosure of this specification.

First Embodiment

FIGS. 4A to 4D are sectional diagrams showing a multi-chip stackstructure and method for fabricating the same according to a firstembodiment of the present invention.

As shown in FIG. 4A, a chip carrier 40 is provided, a first chip group41′ comprising a plurality of first chips 41 is disposed on the chipcarrier 40 in a step-like manner, and a second chip 42 is disposed onthe first chip on the top of the first chip group 41′, wherein the firstand second chips 41, 42 are electrically connected to the chip carrier40 through bonding wires 46.

The first chips 41 and the second chip 42 can be such as memory chipsand a controller chip respectively. Planar size of the second chip 42 issmaller than that of the first chips 41, a plurality of bonding pads 410is disposed at one side on a surface of each first chip 41 and aplurality of bonding pads 420 is disposed at least one side on a surfaceof the second chip 42 (the bonding pads 420 are disposed at severalsides of the second chip 42 in the drawings). The bonding pads 410, 420are electrically connected to the chip carrier 40 through bonding wires46. The chip carrier 40 can be such as a BGA substrate, a LGA substrateor a leadframe. The projecting position of the second chip 42 relativeto the chip carrier 40 can be located in the projecting position of thefirst chip group 41′ relative to the chip carrier 40. Thus, the use areaof the chip carrier 40 does not increase.

As shown in FIG. 4B, a film over wire (FOW) technique is used to stack athird chip 43 on the first chip group 41′ and the second chip 42 with aninsulative film 47 provided therebetween. The insulative film 47 alsocovers part of the ends of the bonding wires of the first chip 41 on topof the first chip group 41′ and at least part of the second chip 42. Theinsulative film 47 has such a thickness that the bonding wires 46 of thefirst chip 41 and the second chip 42 are prevented from contacting thenon-active surface 431 of the third chip 43. The third chip 43 can besuch as a memory chip with bonding pads disposed at one side thereof.The projecting position of the third chip 43 relative to the chipcarrier 40 can be located in the projecting position of the first chipgroup 41′ relative to the chip carrier 40. Thus, the use area of thechip carrier 40 does not increase.

The insulative film 47 is made of an epoxy resin. The insulative film 47is pre-attached to the non-active surface 431 of the third chip 43, aheat source (not shown) is disposed below the chip carrier 40 forheating, the third chip 43 with the pre-attached insulative film 47 isthen stacked on the first chip group 41′ and the second chip 42, as theinsulative film 47 goes into a melting state when heated, the bondingwires 46 of the first chip group 41′ and the second chip 42 areprevented from being damaged by pressure. Thereafter, the heat source isremoved. The insulative film 47 is solidified to support the third chip43 and cover the bonding wires 46.

As shown in FIG. 4C, a fourth chip 44 is further stacked on the thirdchip 43 in a step-like manner. The fourth chip 44 can be such as amemory chip with bonding pads 440 disposed at one side thereof. The sideof the fourth chip 44 having bonding pads 440 offsets a certain distancefrom the third chip 43 so as to expose the region over the bonding pads430 of the third chip 43. The third and fourth chips 43, 44 can beelectrically connected to the chip carrier 40 through a plurality ofbonding wires 46.

As shown in FIG. 4D, an encapsulant 48 is formed on the chip carrier 40to encapsulate the first chip group 41′, the second chip 42, the thirdchip 43 and the fourth chip 44.

According to the above-described fabrication method, the presentinvention further discloses a multi-chip stack structure, whichcomprises: a chip carrier 40; a first chip group 41′ comprising aplurality of first chips 41 disposed on the chip carrier 40 in astep-like manner, wherein the first chips 41 are electrically connectedto the chip carrier 40 through bonding wires 46; a second chip 42disposed on the first chip 41 on the top of the first chip group 41′,wherein the second chip 42 is electrically connected to the chip carrier40 through bonding wires 46; at least a third chip 43 stacked on thefirst chip group 41′ and the second chip 42 with an insulative film 47provided therebetween, wherein the insulative film 47 covers part of theends of the bonding wires of the first chip 41 on the top of the firstchip group 41′ and at least part of the second chip 42, the third chip43 is electrically connected to the chip carrier 40 through bondingwires 46, the insulative film 47 has such a thickness that the bondingwires 46 of the first chip 41 and the second chip 42 are prevented fromcontacting the non-active surface 431 of the third chip 43.

The multi-chip stack structure further comprises a fourth chip 44stacked on the third chip 43, and an encapsulant 48 encapsulating thefirst chip group 41′, the second chip 42, the third chip 43 and thefourth chip 44.

Second Embodiment

FIG. 5 is a diagram showing a multi-chip stack structure and method forfabricating the same according to a second embodiment of the presentinvention. Different from the first embodiment, the present embodimentelectrically connects the first chip on top of the first chip group andthe second chip with the chip carrier through a reverse wire bondingmethod.

As shown in FIG. 5, a stud bump (not shown) is first formed on thebonding pads 410 of the first chip 41 on top of the first chip group 41′and the bonding pads 420 of the second chip 42 by a ball bonding method.Then, the bonding wires 46 for electrically connecting the first chip 41on top of the first chip group 41′, the second chip 42 with the chipcarrier 40 have a ball bond formed to connect the chip carrier 40 andhave a stitch bond formed to connect the stud bump. Thus, the arc heightof the bonding wires electrically connecting the first and second chips41, 42 with the chip carrier 40 is reduced. Thus, the thickness of theinsulative film 47 for disposing of the third chip 43 on the first andsecond chips 41, 42 can further be reduced and accordingly the entireheight of the stack structure is reduced.

In addition, the other first chips, the third and fourth chips can beelectrically connected to the chip carrier through a usual wire bondingmethod or the above-described reverse wire bonding method.

Therefore, the present invention comprises disposing a first chip groupcomprising a plurality of first chips (memory chips) on a chip carrierin a step-like manner; disposing a second chip (a controller chip) onthe first chip on top of the first chip group, wherein the first andsecond chips are electrically connected to the chip carrier throughbonding wires; using a film over wire (FOW) technique to stack on thefirst and second chips a third chip (a memory chip) with an insulativefilm provided therebetween, wherein the insulative film covers part ofthe ends of the bonding wires of the first chip on top of the first chipgroup and at least part of the second chip; and electrically connectingthe third chip to the chip carrier through bonding wires. Thus, thecontroller chip having a size far smaller than the memory chips isprevented from being directly stacked on the memory chips as in theprior art, thereby decreasing height of the entire structure andavoiding wire bonding difficulty and even short circuit problemoccurring if the bonding wires of the controller chip pass over andcontact the memory chips. Further, as the controller chip is disposedbetween the memory chips instead of on top of the memory chips as in theprior art, the required length of the bonding wires and the arc heightare reduced, thereby saving the fabrication cost, avoiding the bondingwire sweep problem and improving the electrical connection quality.Meanwhile, the number of the chip stack layers is reduced, probabilityof delamination occurring on the interface is reduced and the processcomplexity is simplified. Furthermore, as the third chip (memory chip)is stacked on the first and second chips with an insulative filmprovided therebetween by using a film over wire technique and theinsulative film covers part of the ends of the bonding wires of thefirst chip on the top of the first chip group and at least part of thesecond chip, the need of a conventional buffer layer is eliminated,thereby enabling the entire structure to become much thinner.

The above-described descriptions of the detailed embodiments are only toillustrate the preferred implementation according to the presentinvention, and it is not to limit the scope of the present invention.All modifications and variations completed by those with ordinary skillin the art should fall within the scope of present invention defined bythe appended claims.

What is claimed is:
 1. A multi-chip stack structure, comprising: a chipcarrier having a first surface and an opposite second surface; a firstchip group comprising a plurality of first chips disposed on the firstsurface of the chip carrier in a step-like manner, wherein the pluralityof first chips are electrically connected to the first surface of thechip carrier through bonding wires, and the bonding wires are free fromconnecting to the second surface of the chip carrier; a second chipdisposed on a topmost one of the plurality of first chips of the firstchip group, wherein the second chip is electrically connected to thefirst surface of the chip carrier through bonding wires, and the bondingwires are free from connecting to the second surface of the chipcarrier; an insulative film on the topmost one of the plurality of firstchips of the first chip group to cover parts of ends of the bondingwires of the topmost one of the plurality of first chips of the firstchip group and at least a part of the second chip; a third chip having athird surface and an opposite fourth surface, and being stacked on theinsulative film, wherein the third chip is free from contacting thefirst chip group and the second chip, the third surface of the thirdchip contacts the insulative film, the insulative film is free fromcovering the fourth surface of the third chip, the third chip iselectrically connected to the first surface of the chip carrier throughbonding wires, and the bonding wires are free from connecting to thesecond surface of the chip carrier; and an encapsulant formed on thechip carrier to encapsulate the first chip group, the second chip, andthe third chip.
 2. The multi-chip stack structure of claim 1, wherein aplanar size of the second chip is smaller than that of each of theplurality of first chips.
 3. The multi-chip stack structure of claim 1,wherein the plurality of first chips and the third chip are memorychips, and the second chip is a controller chip.
 4. The multi-chip stackstructure of claim 3, wherein each of the plurality of first chips andthe third chip respectively has at least one bonding pad disposed at oneside thereof, and the second chip has at least one bonding pad disposedat at least one side thereof.
 5. The multi-chip stack structure of claim1, wherein the chip carrier is one of a BGA substrate, a LGA substrateand a leadframe.
 6. The multi-chip stack structure of claim 1, wherein afilm over wire (FOW) technique is used to stack the third chip on thefirst chip group and the second chip with the insulative film providedtherebetween.
 7. The multi-chip stack structure of claim 1, wherein theplurality of first chips and the second chip are electrically connectedto the chip carrier by one of a usual wire bonding method and a reversewire bonding method.
 8. The multi-chip stack structure of claim 1further comprising a fourth chip stacked on the third chip in astep-like manner.
 9. The multi-chip stack structure of claim 8, whereinthe third and fourth chips are electrically connected to the chipcarrier by one of a usual wire bonding method and a reverse wire bondingmethod.
 10. The multi-chip stack structure of claim 8, wherein thefourth chip is a memory chip.
 11. The multi-chip stack structure ofclaim 8, wherein the encapsulant encapsulates the fourth chip.
 12. Themulti-chip stack structure of claim 1, wherein the insulative film ismade of an epoxy resin.
 13. The multi-chip stack structure of claim 1,wherein the projecting position of the second and third chips relativeto the chip carrier is located in the projecting position of the firstchip group relative to the chip carrier.